THE MUSCULAR SYSTEM 171
the posterior belly alone is present, and runs to the angle of the mandible.
The stylo-hyoid arises from the styloid process or the styloid region and runs to the hvoid bone in Man
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and all the Apes except the Orang, in which it runs to the wall of the air-sac, thus forming a stylo-laryngeus. Occasionally, however, there is a true stylo-hyoid in the Orang,
The mylo-hyoid (fig. 3lA't arises from the anterior border of the hyoid bone over an extent corresponding to the insertions of the sterno-hvoid and omo-hvoid on
the posterior border. The fibres radiate to be attached to the inner surface of the mandible; but there is no median raphe. The relations are as in Man. Gratiolet (330) described decussation of the fibres in a Chimpanzee, and I observed a similar condition in an Orang. The mylo-hyoid ridge is strongest in Man,
The genio-hyoid muscles in the Chimpanzee (fig. 31 A) run from the lower part of the genial fossa on the back of the symphysis raenti to the upper border of the body and part of the great cornu of the hyoid bone. The relations are as in Man. Between the genio-hyoidei and hyoid bone anteriorly and the genio-glossi posteriorly there is a well-marked bursa. The genio-glossi arise in the bottom of the genial fossa, and the fibres pass upwards into the tongue—some reach the hyoid bone under the hyo-glossus muscles. Between them and the mandible lie the hyo-glossus and stylo-glossus muscles, the sublingual glands, the lingual artery, and the lingual and hypoglossal nerves. Wharton's duct also courses in that area. The mutual relations of these structures are as in Man and the other Apes.